Natural Gas And Oil Producing Activities (Composition Of Net Unevaluated Costs Excluded From Amortization) (Details) - USD ($) $ in Millions	12 Months Ended			108 Months Ended	144 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2015
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Property acquisition costs	$ 274	$ 2,870	$ 42	$ 105	$ 3,291
Exploration and development costs	128	56	29	36	249
Capitalized interest	120	22	11	34	187
Net unevaluated costs excluded from amortization	522	$ 2,948	$ 82	$ 175	3,727
Exploration Program in Canada [Member]
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Net unevaluated costs excluded from amortization	$ 50
Composition of Net Unevaluated Costs [Member] | Exploration Program in Canada [Member]
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Property acquisition costs					16
Exploration and development costs					26
Capitalized interest					$ 8